1. Field of the Invention
This invention relates to a method of and a circuit for starting a sensorless motor, and in particular to such method and circuit capable of improving a starting performance of the sensorless motor in which permanent magnets and driving coils are provided in its rotor and stator respectively, and wherein no sensor like a hall element is provided in detecting a rotor position.
2. Related Art Statement
When a rotor having permanent magnets travels neiborhood of coils of a motor, electromotive force is produced in the motor coils due to variation of a magnetic field. Accordingly, during the rotation of the rotor, even if a position detecting element for rotor is not provided, the position of the rotor is detected by measuring an electromotive force produced in the motor coils.
In a driving circuit of a sensorless motor, a conversion circuit is provided for converting electromotive force produced in the motor coils into signals, and the signals thus converted are used as rotor position signals to operate the circuit.
However, at the time when the motor is intended to start, the motor is in a stopped state and an electromotive force is not yet generated in the coils. Thus, the motor must be started under an unknown condition as to the present position of the rotor. In a stepping step of such a start, an exciting procedure is employed wherein the rotor is caused to rotate in the forward direction in a predetermined sequence of operation irrespective of the relative positions of the stator coils and the permanent magnets of the rotor.
FIG. 10 shows a circuit diagram of a driver of a sensorless motor in the conventional technique. Driving coils of a motor 21 is supplied with an exciting current from an output circuit 23 controlled by a control circuit 22. During rotating of the motor, an electromotive force produced in the driving coils is detected by a detecting circuit 24 and supplied to the control circuit 22 as a rotor position signal. Based on this position signal, the control circuit causes the output circuit 23 to turn on or off and allows the rotor to rotate in the specified direction.
At the time when the starting operation of the motor is intended, the motor is not yet rotated, thus no output is obtained from the detecting circuit 24. And, a switch 27 is connected to a starting circuit 25 which supplies signals to the control circuit 22 for exciting the driving coils in a predetermined sequence of operation. Then, the rotor starts to rotate, and an electromotive force is generated in the driving coils. Now, therefore, the switch 27 is switched to the detecting circuit 24.
However, a predetermined procedure is performed for exciting the coils irrespective of the rotor position, therefore, there arises a case where the rotor moves slightly in the reverse direction in the beginning, or a case where rotating force is small even though the rotor rotates in the correct direction. In such cases, the rotor does not reach a required rotating speed even after a predetermined time lapses. To cope with such failure of starting, a control system as in FIG. 11 is generally used. Namely, after it is instructed to start the control, start/brake signals S/B is switched from a high level to a low level as shown in step S1 so as to begin starting step of the motor. Referring to the circuit of FIG. 10, the switch 27 is connected to the starting circuit 25, and the signals for exciting the stator coils at a predetermined sequence are fed to the control circuit 22. The driving coils of the motor 21 are excited based on these signals.
After executing the specified starting procedure, it is determined in step S2 whether or not the motor is started, by detecting an electromotive force induced in the driving coils (detecting mode). For example, when 90% or above of a predetermined rotating speed has been obtained, the starting is determined as a success.
In success of the starting, the motor is accelerated up to a constant rotating speed.
In failure of the starting, switching S/B signals are changed from Low to High, and a predetermined braking is performed (step S3). This is referred to as a braking step. Thereafter, S/B signals are again changed from High to Low, the starting step being reexecuted.
For example, in case of a driving motor for a magnetic disk of a personal computer, the driving of the magnetic disk is instructed at the same time when the switch of the personal computer is turned on. Generally, starting-up operation of the disk must be completed within about 10 seconds. Accordingly, a counter is prepared to count the number of failures of the starting. If the number of failures reaches the predetermined one, for example 8 times, then the display of the personal computer is caused to display a message of "depress the reset button" or the like for repeating the starting-up operation.
As described above, the starting procedure of the sensorless motor is performed without confirming a position of the rotor. Therefore, it is unavoidable to result in starting failures even in small probability. For example, a starting failure occurs once per 1000 times. If the starting fails, the braking step begins whereby the motor is braked to stop, and returns to an initial condition. Then, the motor is started by the starting step again. Here, the repeated starting steps are independent of each other. Assuming a probability of failure is 1/1000 at each starting mode, and if the starting mode is repeated two times, then a probability of failures is still 1/1000 for each time. Therefore, it is desired to reduce the failures of starting as much as possible. If starting fails, there is a threat that a magnetic head, i.e. a reading and/or writing head, does not float up, and is destroyed.